Pulse generator and modulator system



May 11,1948. w. D. HOUGHTON 2,441,418

PULSE GENERATOR AND MODULATOR SYSTEM Filed Sept. 20, 1945 2 Sheets-Sheet2 NOOUIAT/ON uni/7:: r: 9 fig 4a =5 7;;

fif m V 157E415 I I l N V EN TOR. Y 1441: MM 22 flow/r70 lrroe/viyPatented May .11, 1948 PULSE GENERATOR AND MODULATOR SYSTEM William D.Houghton, Port Jefferson, N. Y., assignor to Radio Corporation ofAmerica, a, corporation of Delaware Application September 20, 1945,Serial No. 617,634-

12 Claims. 1

This invention relates to a pulse signaling system, and moreparticularly to a method of and apparatus for generating pulses ofconstant average frequency and varying their occurrence time withinfixed limits.

An object of the present invention is to enable the phase or timingmodulation of pulses of constant frequency at an audio rate within fixedlimits regardless of the amplitude. of the applied modulation.

Another object is to provide a pulse generator and pulse timingmodulation system suitable for use in a multiplex system in which apulse is produced for each assigned period regardless of the magnitudeof modulation.

A further object is to provide an improved pulse generator which issynchronized and controlled by a triangular wave generator.

A more detailed description of the invention follows, accompanied by adrawing wherein:

Figs. 1, 2 and 3 illustrate three different einbodiments of theinvention, and

Figs. 4a to 4] are curves given in explanation of the operation of theinvention. 7

Throughout the figures of the drawing, the same or like parts arerepresented by like reference characters. 7

Referring to Fig. l in more detail, there is shown a simplified form ofpulse generator and modulator system of the invention-comprising atriode vacuum tube V1 normally biased to be non-conducting and havingits anode connected to a source of positive polarizing potential +3through one winding of a difierentiating transformer T, its grid Gconnected to the outputof a saw-tooth voltage generator, and its cathodeK connected to the anode of a modulator tube V2 and also to ground viaresistor R1. Any appreciable flow of current through tube V1 must bethrough tube V2 since resistor R1 is large in value. In shunt toresistor R1 there is provided a capacitor C1 which is large enough tomaintain the bias required on tubevi but small enough so that it doesnot cause undue attenuation of the higher audio frequencies developedacross tube V2 from the audio modulation, as described later. Themodulator tube V2 comprises a triode vacuum tube to whose grid thereisapplied the audio input modulation from terminal 2 over resistors R4and R3. Resistor R3 is a limiting resistor in the grid circuit of tubeV2 which prevents the grid from being driven positive above the zerovoltage value. Resistor R1Iis a potentiometer which is used tocontrolthe amplitude of the audio modulation voltage applied to thegridof V2. A- resistor R2 in the cathode circuit of tube V2 enables acontrol of the anode resistance of tube V2 and; hence a control of thevalue at which tube V1 conducts on the rise of the sawtooth.

In theoperation of the system of Fig. 1, tube V2 acts as a resistance inthe cathode circuit of tube V1. The value of the resistance offered bytube V2, is a function of its grid-to-cathode volt- 10 age and isdetermined by thesetting of the tap on resistor R2.

- The saw-tooth voltage generator I supplies the grid; of tube: V1 witha saw-tooth wave of positive polarity having a shape similar 'to thatshown in the graph of Fig, 4a. Tube V1 is normally biased to benon-conducting, and is arranged (due to; the setting, of R2) to conductat about the middle ofthe applied saw-tooth voltage wave with no audiovoltage present on terminal 2'. Ob-

viously, any variation in the resistance of tube V2 will result in avariation of the point on the saw tooth voltage wave at which tube V1will conduct, inasmuch as the grid-to-cathode voltage of V1 isdetermined by the average current through V1 and the value of theresistance ofierecl by tube V2. g

When tube V1 conducts, current flows through differentiation transformerT resulting in a pulse of voltage appearing at output terminal 3 incircuit with the secondary windin of the transused in the followingstages.

generated at terminal 8 a positive pulse when tube V1 starts to conductand a negative pulse when tube V1 ceases conducting. Since the startingtime of V1 is the variable, then the correspondingly produced pulse isthe one which 'varies with the modulation. The $01111 line pulse in thecurve of Fig. 4b shows the position of this output pulse relative to theapplied saw-tooth wave, with no audio modulation on input terminal 2.The current cut-off to current limiting region of tube V1 is made to besmall with respect to the amplitude of the applied saw-tooth wavebecause a sharp current change is desired in tube V1, and this region ismoved over the slope of the saw-tooth wave with modulation.

The audio modulation voltage applied to the grid of tube V2 varies theresistance of tube V2 and the bias on tube V1, hence varying thecritical value or time at which tube V1 starts to conduct over the rangeof the applied saw-tooth wave. When the starting point of current flowin tube V1 is varied by the modulation, the occurrence time of theresulting output pulse in transformer T is also varied with respect tothe leading and trailing edges of the applied saw-tooth Wave. When audiovoltages are applied to the grid of tube V2 of sufficient magnitude tocut off the flow of current in tube V2, then the resistance in thecathode circuit of tube V1 becomes the resistance of R1. The value of R1is such that with the flow of current in tube V2 cut off, the tube V1will conduct at the peak of the applied saw-tooth wave. This feature isimportant because it assures an output pulse from. the system for eachapplied saw-tooth wave in the event tube V2 is cut off by excessivemodulation.

As mentioned above, resistor R3 is a grid limiting resistor. Hence theresistance of modulator tube V2 cannot go below a minimum valueregardless of the amplitude of the audio modulation voltage applied tothe grid of this tube. This minimum value is such that tube V1 willconduct at the leading edge of the saw-tooth wave. Consequently,regardless of the amplitude of the applied audio modulation, theposition of the output pulse at output terminal 3 always falls withinthe time interval or duration of the applied sawtooth wave. This isshown by the dotted line pulses in Fig. 4b which illustrate the limitingpositions of the output pulses with extreme positive and negativemodulation voltages.

A more detailed description is as follows: Tube V1 is normally made toconduct at the middle of the linear rise time of the applied sawtoothwave. After V1 startsto conduct, it continues to conduct for theremaining portion of the linear rise time.

When tube V1 conducts, a' charge is stored in condenser C1. This chargeleaks off through the impedance of tube V2 and R1 during the timeinterval during which V1 is non-conducting. Since R1 is large comparedto the impedance offered by tube V2, its effect may be neglected as longastube V2 is conducting. If the value of C1 is very large it willbe seenthat it will require a number of cycles of the sawtooth wave to reachstability, that is, when the charge stored in C1 during each sawtooth isexactly balanced by the decrease in charge during the time interval whenV1 is cut-off. Also, when the voltage on the grid of tube V2 is changedit will require a number of cycles for a new equilibrium condition to beobtained. For example, when the bias on tube V1 is zero and C1 is large,it will require a number of cycles of the applied sawtooth wave beforethe charge stored in C1 is balanced by the decrease in charge providedby tube V2. If after this equilibrium condition is reached and the biason tube V2 is set at a value where tube V1 is conducting at the middleof the linear rise portion of the applied sawtooth wave, the voltage onthe grid of tube V2 is made negative the charge leaked off by tube V2will not be as great as the charge stored when tube'V1 conducts; hencethe voltage across C1 will rise and tube V1 will conduct for a smallerportion of the applied sawtooth. After a number of cycles, a newequilibrium will be established with tube V1 starting to conduct at ahigher or later point on the sawtooth, thus producing a pulse in theoutput winding of transformer T at a later time than when the bias ontube V2 was zero. Likewise, if the bias on tube V2 is made positive, thecharge leaked ofi by tube V2 will be greater than that stored when tubeV1 conducts and hence tube V1 starts to conduct at a lower point on thesawtooth. After a number of cycles, a new equilibrium point is reachedwhere tube V1 is conducting for a greater portion of the appliedsawtooth than when the bias on tube V2 was zero.

It will thus be seen that if condenser C1 were very large it wouldrequire a large number of cycles of the sawtooth wave before equilibriumwould be reached. It is common practice to make the pulse carrier (thesawtooth frequency) two or three times the highest audio frequency andit is therefore necessary to make condenser C1 of such a value thatequilibrium may be established within this time. C1 must have a minimumsuch that with the type of tube used for V2, a bias will be developedsuflicient to cause tube V1 to conduct at the middle of the linear risetime.

Fig. 2 shows modification of the system of Fig. 1. In Fig. 2, audiomodulation voltage from input terminal 2 is first applied to the grid ofa cathode follower or cathode amplifier tube V3 whose cathode isconnected to the cathode of tube V2. Both tubes 'V2and V2 use the samecommon cathode resistor R2. Any variation in current through vacuum tubeV2 in response to audio modulation from input terminal 2 causes avariation of the cathode voltage of tube V2. Because the grid voltage oftube V2 is fixed, this variation in cathode voltage changes the averageanode resistance of tube V2 and hence the cathode bias of tube V1.

Fig. 3 illustrates a third modification of the invention in which adirect current voltage is impressed on the grids of tubes V1 and V2 bymeans of the bleeder circuit composed of resistors R5, R6 and R7. Hereagain, the audio modulation is applied to the cathode follower tube V3,but the modulation voltage is now inserted in the grid-to-cathodecircuit of tube V2 across resistor R2; A degeneration resistor R8 in thecathode of V2 makes the anode resistance of tube V2 vary in a linearmanner with applied audio voltage. The saw-tooth voltage from generatorl is now applied to the grid of tube V1 through a condenser C2. Theconnections are such that there'is a direct currentvoltage' applied tothe grid of tube V1 to ether with the saw-tooth wave. This raisesftheaverage voltage on the grid of tube V1 and as a result raises the directcurrent anode voltage of tube V2. This allows tube V2 to work at a moresuitable value of anode voltage. The amplitude of the applied saw-toothvoltage, however, remains the same in both conditions.

The maximum advance position in time of an output pulse-from the systemof Fig. 3 due to negative modulating voltage is determined by theminimum anode resistance of tube V2 (maximum conduction condition) whichoccurs when tube V3 is cut off, and the actual value of this minimumanode resistance of tube V2 is set by the values of resistors Ba and R2.When a maximum positive modulating voltage is applied to tube V3, itscurrent will cut 01? tube V2 and leave resistor R1 as the biasingresistor for tube V1. Thus the flow of current through resistor R1 whentube V2 is cut oif, automatically biases tube V1 so that it yields auseful pulse at the top position of any saw-tooth wave applied to thegrid of tube V1.

The system; of the invention can be used in a pulsemultiplex'communication system wherein different transmitting -channelsvhave individual pulse generator and modulator circuits; like thoseshownii-n Figs. .1, .2 on3. :Whcn 'usedinamultiplea-system, thedifferent pulse generator circuits are supplied with separate saw-toothvoltages, each displaced in time so that the start of one --saW-toothfellows theideca of another saw-tooth limits of the pulse range when achannel is fully modulated. Only three channels are represented in Figs.through 4], although more channels .may be used. One suchpulse multiplexsystem utilizing the present invention is described in my copendingapplication Serial No. 608,957, filed August 4, 1945. v

One advantage of using the invention in va pulse multiplex system liesin the assurance that a pulse will be generated in. each channel foreach assigned period regardless of the presence of excessive modulation.This prevents .crossrno'dulation or cross-talk in the channels which mayotherwise occur due to a change in power levels throughout the system ifa pulse were not passed on the peaks ofmodulation by a channel. to

.The invention is not limited to th precise detailsshown in thedrawingsince various changes can be made by the substitution ofequivalent elements without departing from the spirit and scope of theinvention. As an illustration, the

transformer T can bev replaced bya differentiating coil one terminal ,ofwhich can be con- .nected to the anode of V1, and the other terminal.connected to the positive terminal of a source ofuni'directionalpotential. In this case, the out- .put would be obtainedfrom the anode of tube V1 via a, coupling condenser to .rem'ovethedirect current from the output. With such an arrangement, the pulsewhich is varied with modulation would .be negative, while theunmodulated pulse wouldbe positive. Alternatively, the transformer Tcould be replaced by a center-tappedlinductance coil one terminal ofwhich can be connected to the anode of V1 while the other terminal isconnected to +13. In this last case, the output pulse (pulse which isvaried with modulation) would be positive and would be coupled toutilization circuits via a coupling capacity to remove direct currentfrom the output.

If a variable width output pulse is desired, the transformer'T canbereplaced bya resistor of suitable value to provide a desired build-upand decay of the pulse. The variable width pulse generated in this eventwould be negative in polarity.

The invention can be used wherever there is need for a pulse generatorcontrolled by a sawtooth wave, with or without the modulation feature.For example, the use of tubes V1 and Vi and associated circuit elementscan be used in a radar system where the saw-tooth sweep wave for thecathode ray device is generated simultaneously with the outgoing wave.This sawtooth sweep wave can be applied to the, grid of tube V1. Bycalibrating resistor R2 in distance and adjustingRz manually, it ispossible to produce amarker pulse-which can be made to occur at the sameinstant as the echo-wave is received.

For this particular application of the invention,

there'is no modulation applied to tube "V2.

The-term ground used in the appended claims is deemed to include anypoint orsu-rface of zero potential for alternating current or directcurrent, and may be considered a point of reference potential. v

What is claimed is:

1. A pulse generator comprising firs and sec- 0nd vacuum tubes eachhaving an anod acath- .ode, and a controlelectrode, a source of tringular waves coupled to'the control electrode of said first tube, a.differentiating circuit connected between the anode of said first tubeand the positive terminal of a source :of unidirectional potential, aconnection from the cathode of said first tube to the anode of saidsecond tube, aresistor ,in the cathodecircuit :of said second tube, aconnection between the .grid .ofsaid second tube ands i cathoderesistor, and a resistor -shuntedby a capacitor connected-between the:anodeiof said second tube and that terminal of the cathode resistorfarthest removed from the .calthod of said second tube, said second tubefunctioning as .a resistance in the cathode circuit of said first tube,the value of said capacitor :being such that said first tube is normallybiased to the anode current cut-off condition, the efiectiveanoderesistance of said second tu-be determiningthe point on the appliedtriangular wave :at which said first tube conducts, and an outputterminal cou- -pled to said differentiating :circuit.

'2. A pulse generator comprising first-and second vacuum tubes eachhaving .an anode, .a cathode, and a control electrode, a :source oftriangular-waves coupled to the control electrode of said first tube, adifferentiating transformer connected between the anode of said firsttube and the positive terminal of a source of unidirectional potential,a connection from the cathode of said first tube to the anode of .said

second tube, a variable resistor in the cathode circuit of said:second'tube, a-connectionbetween the grid of saidsecond tube andsaidcathode resistor, and a--resistor connected between I the anode of saidsecond tube and that terminal of the cathode resistor farthest removedfrom the cathode of said second tube, said second tube functioning as aresistance in :thecathode circult of said first tube, means-normallybiasing said first tube to the anode current .cut-oii con- 'diticn, theeffective anode resistance-of said second tube determining the point onthe applied triangular wave at which said first tube "conductsjand anoutput terminal coupled to said differentiating transformer.

3. A pulse gene-rator comprising first and sec- -ond vacuu-mtubes eachhaving "an anode, a

cathode, and a control electrode, asource of -.-ma1'anode resistance ofsaid second tube being such and the circuit components having-suchvalues and so arranged that said first tubeis normally biased to theanode current cut-oif condition in the absence of an applied triangularwave and said "firsttube conducts lat -a :point inmtermediate the endsof :the iorward slopeuof the coupled to said difierentiati'ng' circuits1 j applied triangular wave} and; an output terminal 1. A pulsegenerator comprising first and second vacuum tubes each having an anode,a

cathode, and a control electrode, a source of tri- 1 angular wavescoupled to the control electrode tube, a' resistor connected between'thecathode of 'said second tube and ground,-a connection from the controlelectrode of said'second'tube; to

a variable tap on said'last resistonfa resistor shunted by a condenser'connectedlb'etweenfthe anode of said second tubezandground, the anoderesistance of said second tube being such and said condenser having suchvaluethat said firsttube is normally .biased to cut-:ofi. in ,theabsenceof an applied triangular wave, and an output terminal coupled to saiddifferentiating circuit.

5. A pulse generator comprising first and',sec- 0nd vacuum tubes each"having an anode, a cathode, and a control electrode, a source oftriangular waves coupled to the control electrode of said first tube, adifierentiating circuit connected between the anode of said'first tubeand the positiveterminal of a source of unidirectional potential, aconnection from the cathode of said first tube to the anode of saidsecond tube, a resistor. connected between the cathode of said secondtube and ground, a resistor connected between the control electrode ofsaid second tube and ground, a resistor shunted by a condenser connectedbetweenthe anode of said second tube and ground, the normal anoderesistance of said second tube being such and said condenser having suchvalue that said first tube is normally biased to the anode currentcut-off condition in the absence of an applied triangular wave and saidfirst tube conducts at a point intermediate theends of the forward slopeof the applied triangular wave, and means for varying the anoderesistance of said second tube in accordance with a modulating voltageto thereby vary the current conducting point of said first tube.

6. 'A pulse generator comprising first and sec 1 0nd vacuum tubes eachhaving an anode, a

cathode, and a control electrode, a source of triangular waves coupledto the control electrode of said first tube, a difierentiating circuitconnected between the anode of said first tube and the positive terminalof a source of unidirectional potential, a connection from the cathodeof said first tube to the anode of said second tube, a

resistor connected between the cathode of said second tube and ground, apotentiometer having one terminal connected to ground and anotherterminal connected to a source of modulating voltage, a connectionincluding a grid limiting resistor from the grid of said second tube toa said-first tube, a difierentiating circuit connected between theanodeof said first tube and the positive-terminal of a source ofunidirectional potenti'aha connection from the cathode of said firsttube-'tothe anode of said second tube, a resistor connected between thecathode of said second tube and ground, a resistor shunted by aconde'nserconnected between the anode of said second tubeand ground, aresistance network connected between said positive terminal and ground,and connections from different points on saidresist'ance network tothecontrol electrodes of said first and second tubes, the value of saidcondenser and the normal anode resistance of said second tube being suchthat said first tube is normally biased to the anode current cut-offcondition in the absence of an applied triangular wave and said firsttube conducts at a point intermediate the ends ofthe forward slope ofthe applied triangular wave, and an output'terminal coupled to saiddifferentiating circuit.

8; A pulsegenerator'comprising first and second vacuum tubes each havingan anode, a

cathode, and a control electrode, a source of triangular waves coupledto the control electrode of said first tube','a differentiatingcircuitconnected between the anode of said first tube andthe positiveterminalof a source of unidirectional potentiaL'a connection from thecathode of said first tube to the anode of said second tube, a resistorconnected between the cathode of said second tube and'ground, a resistorconnected between the controlelectrode of-saidsecond tube and ground, aresistor shunted by a'condenser connected between the anode of saidsecond tube and ground, the normal anode resistance of said second tubebeing such that said first tube is normally biased to the anode currentcut-oil" condition in'the absence of an applied triangular wave and saidfirst tube conducts at a point intermediate the ends of the forwardslope of the applied triangular wave, meansfor varying the anoderesistance of said second tube in accordance with an audio modulatingVoltage to thereby vary the current conducting point of said first tube,said means including a third vacuum tube whose cathode is connected tothe cathode of said second tube and whose control electrode is connectedto a source of audioivoltage, and an output terminal coupled to saiddifierentiating circuit.

9, A pulse generator comprising first and second- 'vacuum tubes eachhaving an anode, a cathode, and a control electrode, a source ofsawtooth waves coupled to the control electrode of said first tube, adifferentiating circuit connected between the anode of said first tubeand the positive terminal of a source of unidirectionalpotential, aconnection from the cathode of said first tube to the anode of saidsecond tube, a resistor in the cathode circuit of said second tube, acontube determining the point on the applied sawtooth wave at which saidfirst-tube conducts, means for varying the anode resistance of saidsecond tube in accordance with an audio voltage to thereby vary thecurrent conducting point of said first tube, said means including acathode follower tube whose cathode is connected to the a connectionfrom the cathode of said first tube to the anode of said second tube, aresistor connected between the cathode of aid second tube and ground, aconnection from the control electrode of said second tube to a variabletap on said last resistor, a resistor shunted by a condenser connectedbetween the anode of said second tube and ground, the value of saidcondenser and the anode resistance of said second tube being such thatsaid first tube is normally biased to cut-oflf in the absence of anapplied triangular wave, means for varying the anode resistance of saidsecond tube in accordance with an audio voltage to thereby vary thecurrent conducting point of said first tube, said means including acathode follower tube whose cathode is connected to the cathode of saidsecond tube and whose control electrode is connected to a source ofaudio voltage, and an output terminal coupled to said diiferentiatingcircuit.

11. A pulse generator comprising first and second vacuum tubes eachhaving an anode, a cathode, and a control electrode, a source oftriangular waves coupled to the control electrode of said first tube, animpedance network connected between the anode of said first tube and thepositive terminal of a source of unidirectional potential, a connectionfrom the cathode of said first tube to the anode of said second tube, aresistor in the cathode circuit of said second tube, a connectionbetween the grid of said second tube and said cathode resistor, and aresistor shunted by a condenser connected between the anode of saidsecond tube and that terminal of the cathode resistor farthest removedfrom the cathode of said second tube, said second tube functioning as aresistance in the cathode circuit of said first tube, the value of saidcondenser being such that said first tube is normally biased to theanode current cut-off condition, the effective anode resistance of saidsecond tube determining the point on the applied triangular wave atwhich said first tube conducts, and an output terminal coupled to saidimpedance network.

12. A pulse generator comprising first and second vacuum tubes eachhaving an anode, a cathode, and a control electrode, a source oftriangular Waves coupled to the control electrode of said first tube, adifferentiating circuit connectedibetween the anode of said first tubeand the positive terminal of a source of unidirectional potential, aconnection from the negative terminal ofgsaid source of unidirectionalpotential to a point of reference potential, a direct connection fromthe cathode of said first tube to the anode of said second tube, aresistor between the cathode of said second tube and said point ofreference potential, a connection between the grid of said second tubeand said cathode resistor, a resistor connected between the anode ofsaid second tube and said point of reference potential, said second tubefunctioning as a resistance in theba-tliode circuit of said first tube,means for normall'gt biasing the first tube to the anode current cut-oncondition, a source of audio modulating voltage cupled to said secondtube, and an output terminal coupled to said differentiating circuit.

WILLIAM D. HOUGHTON.

